Contact Author:
Prof. LUIS E. HERRANZ
luisen.herranz@ciemat.es
P:34-913466219
F:34-913466233
Avda. Complutense, 22
Nuclear Safety Unit (Bldg. # 12)
MADRID, MADRID 28040
SPAIN
EXERGY AND ECONOMICAL ANALYSIS OF DIRECT AND INDIRECT BRAYTON CYCLES based on PBMR REACTOR
L.E. Herranz (CIEMAT), J.I. Linares (COMILLAS), B.Y. Moratilla (COMILLAS)
Generation IV systems are devised to achieve sustainability and to improve economic competitiveness of Nuclear Power Plants. They intend to respond to energy world demand in a broader way than current nuclear technology. Systems are being designed to cover a wider range of power outputs, from small units to large ones. In addition, applications other than electricity production are being targeted like hydrogen production, desalination and/or cogeneration.
Two major pillars of power generation sustainability are: optimization of natural resources exploitation and minimization of waste production. Both of them could be substantially enhanced by using power cycles capable of attaining high thermal efficiencies. No less important, an optimum Balance Of Plant (BOP) could also result in a reduction of the nuclear kWh cost that would contribute to make nuclear energy even more competitive. Therefore, the BOP is considered a key aspect of Generation IV systems.
This paper investigates suitable options for the BOP of a PBMR-based nuclear reactor. The Eskom proposal for the South Africa PBMR (a direct Brayton cycle) is analysed and compared to a potential indirect Brayton cycle, and the major thermal and economical advantages and disadvantages are highlighted. By means of an exergy analysis the most important components of the cycle are identified and the power cycle response to changes in operational parameters and variables can be well understood. A sensitivity analysis completes the picture.
The exergy analysis shows that heat exchangers are responsible for plant behaviour, while through an optimization of turbo-machinery a significant and nearly uniform enhancement of thermal performance could be accomplished. The Intermediate Heat Exchanger (IHX) is demonstrated to have a merely side effect. Thus, the effectiveness loss caused by using a shell-and-tube HX instead of using a fin-plate one, hardly affects the thermal behaviour whereas it reduces significantly the capital cost of the electricity generated.
The total levelized generation cost of both cycles has been assessed from open data in literature and estimates of the reactor investment. The indirect cycle is 25% more expensive than the direct one, but it has the possibility to operate in load-follow mode maintaining the cycle efficiency and generation costs. Such a flexibility is an advantage under market conditions. In addition, according to the exergy analysis, a less demanding performance of some components could notably decrease their capital costs making both cycles more similar.
In summary, once optimized according to the results of the exergy analysis, the indirect cycle baseline has been shown to be capable of working in a load-follow mode without loosing any thermal efficiency or rising generation costs. Thus, the indirect cycle proposed exhibits interesting features for the operation of a nuclear power plant under foreseen challenges in the energy generation business.